In recent years, IEEE (Institute of Electrical and Electronics Engineers) 802.11n (draft version 2.0 and newer version) specification is proposed (drawn) as a standard which intends to increase capacity and communication speed of a wireless LAN (Local Area Network). IEEE802.11n realizes effective communication speed over 100 Mbps by using 2.4 GHz frequency band and 5 GHz frequency band. IEEE802.11n adopts a channel bonding technique in which a plurality of channels (bandwidth used for communication) is bonded as a technique to satisfy the above-mentioned specification. According to the channel bonding technique, a plurality of channels is bonded to broaden the bandwidth used simultaneously for radio communication from 20 MHz to 40 MHz, and consequently higher speed communication is realized.
While only bandwidth of 20 MHz (one channel) has been licensed by regulation of the radio law in Japan, the radio law has been amended in recent years to broaden the bandwidth used simultaneously for the radio communication from former 20 MHz to 40 MHz. That is, the channel bonding technique has become available in Japan.
In the channel bonding technique used by a general wireless LAN device based on the IEEE802.11n specification, two channels adjacent to each other are always bonded (for example, refer to Japanese Patent application Laid-Open No. 2008-507234).
Meanwhile, as the wireless LAN is widely spread in recent years, mutual interference between the wireless LAN devices causes new problems. That is, the mutual interference degrades throughput. Accordingly, various kinds of techniques which can reduce the mutual interference between the wireless LAN devices are proposed. For example, Japanese Patent Application Laid-Open No. 1996-84148 discloses a technique in which radiation in an unnecessary direction is suppressed through giving an antenna of a mobile station directivity and consequently interference with other station is suppressed. Japanese Patent Application Laid-Open No. 2004-274765 discloses a technique to select a channel having low-level interference. Japanese Patent Application Laid-Open No. 2007-96786 discloses that an appropriate criterion is made when a channel is selected. Japanese Patent Application Laid-Open No. 2007-295541 discloses a terminal device directly communicates with other terminal device using a different channel from a channel which is used for communication with a base station. Japanese Patent Application Laid-Open No. 2002-217917 discloses a method in which a multicast signal sent from a mobile station is analyzed. Japanese Patent Application Laid-Open No. 2005-210616 discloses a method in which communication starts shortly after an operation for communication.